The present invention relates, in general, to a microfabrication process for making enclosed structures, and more particularly to a process for fabricating microfluidic tunnels, cavities, channels and similar structures suspended above a substrate or embedded below the surface of a substrate such as a single crystal silicon wafer, to the tunnels, cavities and related enclosed microstructures so fabricated, and to microfabricated systems or devices incorporating such enclosed structures.
Microfabrication holds the promise of dramatically improving the efficacy and cost of fluid processing systems. In general, microfabrication enables the production of large numbers of nearly identical devices, and as the number of devices produced by this technique increases, the cost per device decreases. As technology improves, which is inevitable considering the investment in and success of the semiconductor industry, the ability to fabricate microfluidic systems will also improve.
Early developments in micromechanics successfully led to the fabrication of microactuators utilizing processes which involved either bulk or surface micromachining. The most popular surface micromaching process used polysilicon as the structural layer in which the mechanical structures were formed. In a typical polysilicon process, a sacrificial layer was deposited on a silicon substrate prior to the deposition of the polysilicon layer. The mechanical structures were defined in the polysilicon, and then the sacrificial layer was etched partially or completely down to the silicon substrate to free the structures. Moving rotors, gears, accelerometers and other structures were fashioned through the use of this process to permit relative motion between these structures and the substrate. This process relied on chemical vapor deposition (CVD) to form the alternating layers of oxide and polysilicon and provided significant freedom in device design. However, CVI) silicon was usually limited to layers no thicker than one or two micrometers.
An alternative was the use of bulk micromachining wherein a silicon substrate was etched and sculpted to leave a structure. This was typically done using wet chemical etchants such as EDP. However, such processes were dependent on the crystal orientation within the silicon substrate, with the result that the process was difficult to control. Accordingly, wet etch processes were not applicable to small structure definition.
To overcome the disadvantages of the forgoing processes, a reactive ion etching (RIE) process for the fabrication of submicron, single crystal silicon, movable mechanical structures was developed and is described in U.S. Pat. No. 5,198,390, assigned to the Assignee of the present application. That process utilized multiple masks to define structural elements and metal contacts, and permitted definition of small, complex structures in single crystal silicon. This process required a second lithography step which was difficult to apply to deeper structures because of problems in aligning the second mask, and accordingly a single-mask low temperature, self-aligned process for fabricating micron scale microelectromechanical (MEM) structures was developed and is described in U.S. Pat. No. 5,719,073, the disclosure of which is hereby incorporated herein by reference. The process described in the '073 Patent is a dry bulk micromachining process which uses reactive ion etching to both define and release structures of arbitrary shape having minimum dimensions of about one or two micrometers (i.e., micron-scale) and to provide defined metal surfaces on the release structures, as well as on stationary interconnects, pads, and the like. The single mask process permits fabrication of complex shapes, including triangular and rectangular structures as well as curved structures such as circles, ellipses and parabolas, for use in the fabrication of fixed and variable inductors, transformers, capacitors, switches and the like. The structures are released from the underlying substrate in the fabrication process, allowing them to be moved with respect to the substrate.
Although the single mask process described in the '073 Patent had numerous advantages and permitted fabrication of a wide variety of microelectromechanical structures on the surface of substrates, it was recognized that micron-scale structures located beneath the surface of the substrate would have a wide range of applications, and accordingly a process for producing such structures was described in the aforesaid co-pending application Ser. No. 08/867,060. As described therein, a process for fabricating enclosed micron-scale structures having minimum dimensions on the order of ten microns or less was developed. The enclosed structures included micron-scale cavities, tunnels, or other subsurface enclosures for carrying fluids such as gases or liquids under controlled conditions for use in electrophoresis, for use as ink jet nozzles, and the like. The microfabrication process utilized in that application was compatible with existing integrated circuit processes and structures so that it could be carried out on chips or wafers containing integrated circuits, and in addition was highly controllable to permit the fabrication of enclosed tunnel-like structures in a substrate and in a wide range and variety of configurations.
Microstructures in accordance with the '060 application were capable of use in biological and chemical synthesizers and analyzers, in gas sensors, ink dispensers for printers, and pressure sensors, in display devices, in optical applications, and the like. The subsurface structures were embedded in a substrate, or could be suspended by way of released beams, and could be fabricated in a wide range of cross sectional sizes and shapes. In accordance with that application, the subsurface structures were fabricated in single crystal silicon utilizing the SCREAM process described in the aforesaid U.S. Pat. No. 5,719,073. Thus, an isotropic silicon etch was used to produced a subsurface cavity beneath and along via channels to produce subsurface cavities conforming to the location of the via channels. The diameter, or cross-sectional size, of each cavity was determined by the duration of the etch. Thereafter, an oxide layer was deposited on the top surface of the substrate to fill in and cover the via channels to seal the entrances to the respective subsurface cavities. This formed enclosed subsurface structures having shapes and dimensions determined by the via channels and the etch duration. Thereafter, if desired, the subsurface structure could be located in a released beam by etching around the location of the subsurface cavity.
Microfabrication techniques applied to the construction of fluid processing systems have yielded a number of advantages over conventional fluid systems. Foremost are the benefits derived from the miniaturization of the system, including making systems more portable, making more efficient use of expensive reagents or limited samples, and by providing increased speed and sensitivity of chemical analyses. The ability to fabricate complex miniature systems also enables fundamental studies of fluid behavior on a microscale. Further, the tendency toward integration of microfabricated systems promises high degrees of automation and the parallel nature of the process and the capability to produce high volumes of devices leads to the ability to easily scale processing speed and throughput. Finally, the prospect of large numbers of inexpensive systems allows the realization of disposable systems to avoid contamination.
The foregoing and other advantages have led to an increase in research in microfluidics, with a number of products depending on fluidics having been developed; most notably the ink jet printer. Areas of additional research include the behavior of fluids on a microscale, fabrication of microfluidic components such as pumps and valves, and development of a wide range of sensors for parameters such as pressure, temperature, flow rate and pH. Further, such research has led to the development of systems such as micro total analysis systems, and the adaptation of analytical methods such as capillary electrophoresis and PCR.
The greatest benefits of such developments will be derived from the implementation of microfluidic systems, as opposed to isolated components. Only from a systems standpoint will the cost benefits be realized and automation be possible. For example, unless sample preparation is integrated into the system, the advantages of microfabricated fluidic devices will be limited in terms of cost, time and throughput. Thus, a number of integration issues must be taken into account when developing processes for fabricating fluidics systems, for it will be important to integrate such systems with electronic controls, with sensing, with feedback and logic and the like. Further, standard analytical techniques such as optical excitation and interrogation of chemical reactions must be considered, and integration with existing MEMS technology must be considered.